Gravimetric sensing is a powerful tool for used in such applications as geological surveying, oil field exploration, earthquake detection, homeland defense, and shipping container shock detection. A gravimeter is an instrument used measuring a local gravitational field. A gravimeter is a highly sensitive type of accelerometer, specialized for measuring the constant downward acceleration of gravity. Gravimeters have better sensitivity than a conventional accelerometer, however, which enables them to measure very tiny fractional changes within the Earth's gravity. Such small changes in local gravity can be caused by, among other things, a geologic structure, a mass of highly dense material (e.g., nuclear material and its storage container), or the shape of the Earth.
An absolute gravimeter provides an absolute value for gravity local to a position. A typical absolute gravimeter comprises a mass that is propelled upward to an apex, from which it subsequently free-falls. This is normally performed in a vacuum to mitigate the effects of air friction. Acceleration is determined based on the characteristics of the free-fall of the mass. In some prior-art gravimeters, the mass includes a retroreflector that terminates one arm of a Michelson interferometer. By counting and timing the interference fringes, the velocity and acceleration of the mass during free-fall can be determined. In some cases, the system measures both upward and downward motion of the mass, thereby enabling the cancellation of some systematic measurement errors.
Two or more gravimeters can be used in unison to provide a relative measure of gravity over a region. Two- or three-dimensional mapping of a gravitational field can provide a great deal of information about sub-surface structure and materials. A sensor that is capable of precisely mapping the gradients in the gravitational field can offer a high degree of precision about the density profiles of nearby geological formations, such as mineral deposits or subterranean oil fields.
The most common type of relative gravimeter is spring based. A spring-based relative gravimeter is basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be determined. The spring must be carefully calibrated, however. This is typically done by placing the instrument in a location with a known gravitational acceleration.
The high-sensitivity of a gravimeter makes it susceptible to extraneous vibrations. Numerous approaches have been used to attempt to mitigate the deleterious effects of such vibrations. For example, many gravimeters include integrated vibration isolation. Unfortunately, such isolation requires complex and expensive infrastructure and affords only partial isolation. Sophisticated post-measurement signal processing has also been applied to reduce the noise due to vibrations and improve signal-to-noise ratio (SNR). This requires, however, a highly developed model of the noise sources and also adds to the cost and complexity of the gravimeter system. Alternatively, since some applications do not require gravity measurements at high speed, attempts to improve SNR have included time-averaging the output of the device. Although time averaging offers improvement in gravimeter sensitivity, it precludes the use of such systems in many applications.